A method and machine for aligning flexographic printing plates on printing cylinders are the subject of the present invention.
As is known, flexible plates or cliche are used in the printing sector for packaging, for example flexible packaging, PVC wrapping, paper bags, napkins, tablecloths and boxes for the paper and cardboard industry. On these flexible plates or cliche there are raised parts suitable for realising the design, or part of the design that needs to be printed.
In particular, if there is the need to print images with various colours, it is necessary to include a variety of plates, each with just the part of the design to be created in the relative colour raised. This plurality of plates is fixed individually on separate cylinders or printing sleeves to transfer the relative colour separately, which is deposited on the raised parts, to the strip, for example of paper, which runs or rolls on these rotating cylinders.
In order for the different plates to make up one single multi-coloured image, it is necessary for them to be well aligned or, in other words, for them to work on the same part of the strip or film of paper to be printed. To do this, they must be mounted on the relative printing cylinder in the same position.
One of the requirements for obtaining sharp images and avoiding errors of the various colours of one same image overlapping is therefore to make the positions of the different plates to be used to create this image on the relative printing cylinders coincide exactly. In other words, the position of the plates on the cylinders must always be the same.
In order for them to be aligned, the plates currently used usually present specific references at the ends, for example in the shape of small crosses, circles or squares, which must coincide with alignment systems planned on plate mounting machines. These alignment systems generally comprise light signal projectors, for example laser rays, microscopes, optical scanners, and telecameras.
For example, patent EP 0 329 228 describes a device for positioning a flexible printing plate with at least two position marks on one printing cylinder, comprising two light sources that are movable in a direction parallel to the axis of the printing cylinder to project two light signs on said printing cylinder, a memory for storing the coordinates of the position signs on the printing plates, and a control unit for controlling movement of the light sources in relation to the coordinates of the position marks present in the memory.
In practice, once the light sources have been suitably positioned according to the theoretic coordinates or coordinates required present in the memory for projecting corresponding light signs on a printing cylinder, an operator manually positions a plate on the cylinder so that the position marks on this coincide with the projected light signs.
U.S. Pat. No. 3,186,060 describes a device for mounting curved and rigid printing plates on printing cylinders. Alignment of the plates is carried out by positioning two lamps so that the signs projected by these coincide with two signs foreseen on a printing plate. The subsequent plates are positioned so that their reference signs coincide with the signs projected by the lamps.
U.S. Pat. No. 4,448,522 relates to a method and automatic control device for aligning printing cylinders of a printing machine. According to this method, an operator inserts the theoretic coordinates into a control computer, which should have the reference marks on the printing plates mounted on the cylinders. Two optical scanners are commanded to position themselves in said theoretic coordinates and carry out a “brush” along each printing plate until they intercept the relative reference marks. After measuring the error between the theoretic position and the real one, actuator devices connected to the cylinders are driven to correct the position of the cylinders until said error is eliminated.
The use of telecameras as devices for alignment is currently very popular thanks to their flexibility. It is in fact possible to create a virtual reference figure on the telecamera video of a desired form and then position the plates on the cylinder so that the reference marks, which are present on these, are framed by the telecameras and coincide with the figure of reference on the video.
All of the equipment and methods referred to above nonetheless require the presence of suitable reference marks on the plates and extremely high mechanical precision of this equipment. In particular, there must be perfect alignment between the axis on which the alignment instruments move, the axis of the printing cylinder and the axis passing through the reference marks of the plates.